A nuclear reactor core contains in pertinent part a plurality of fuel bundles, each bundle comprising, for example, an 8.times.8 array of elongate fuel rods disposed in an elongate metallic structure, such as a fuel channel. These structures are disclosed in ample detail in U.S. Pat. No. 3,431,170 (Lass et al.; issued Mar. 4, 1969; assignee: General Electric Company), which is hereby expressly referred to and incorporated herein.
In a common configuration of a reactor core, each channel has a generally square cross section defined by opposite pairs of parallel walls which contain the fuel rods therebetween. The channels may be organized into modules, each module including four channels as well as an elongate control rod having a cruciform cross section which separates the channels within the module.
It is known by those skilled in the art that fuel channels for use in a nuclear reactor core must be fabricated from materials having certain required properties. These include low absorption of neutrons emanating from the UO.sub.2 fuel, high corrosion resistance, and high mechanical strength to withstand stresses in the reactor core. Zirconium and alloys thereof, e.g., the commercially available Zircaloy-2 and Zircaloy-4 alloys, are known to provide these properties and are widely used in the fabrication of fuel channels for nuclear reactor cores. Zircaloy-2 contains about 1.5 percent tin, 0.15 percent iron, 0.1 percent chromium, 0.5 percent nickel and 0.1 percent oxygen. Zircaloy-4 contains substantially no nickel and about 0.2 percent iron, but is otherwise similar to Zircaloy-2.
In the fabrication of fuel channels, the flat material is shaped into a box form and sometimes uniformly heated to temperatures above 1500.degree. F. to provide increased resistance to corrosion. Experience has shown that exposure to radiation within the reactor core causes the fuel channels to grow in length as a result of the effect of the radiation on the microstructure of the Zircaloy. This can cause unwanted bowing when the growth is non-uniform.
Typically, each channel may have a length on the order of thirteen feet and it is secured on at least one end of the fuel bundle which it surrounds. If the bowing action occurs in the direction of the control rod and the bowed channel makes actual contact with the control rod, a malfunction of the reactor may result. Thus, all such bowing is carefully monitored in existing nuclear installations and drastic and costly shutdown procedures are initiated to prevent the fuel channel or the control rods from interferring with reactor operation.